The state of the art and the present invention will essentially be discussed hereafter in the context of a medical or veterinary application of the present invention, where the illuminated area is an operating room or another medical or veterinary intervention room, for example, a dermatological intervention room. The observation field or operation area then is a portion of a patient's body, with locations which have fixed fluorescent elements indicative of specific physiological characteristics. For example, on the skin, areas affected by disease or attacked by parasites are likely to fix more fluorescent elements than other skin areas. Then, an observation of the fluorescent elements (or markers) can reveal these areas. Similarly, in an operation area, a failing portion of an organ, for example, cancerous cells, is likely to specifically fix fluorescent elements. In both cases, the patient and the room in which he is must be illuminated so that the practitioner can observe the diseased area and possibly intervene, and the fluorescence light must also be observed. More specifically, in an operating room where a surgeon must operate, it is important for the operation area to be particularly well illuminated.
A problem which arises for the analysis of fluorescent areas is that, generally, the fluorescence light intensity is very low as compared with the normal ambient lighting. In the fluorescence wavelength range of a specific fluorescent element, the fluorescence light often has an intensity from 106 to 107 times lower than the ambient light in the concerned spectral range. It should be noted that this problem of an intensity difference between the white light intended to illuminate the scene to provide a good visibility to the naked eye, and the fluorescence light observed by a camera, does not arise in situations where a white light image and a fluorescence image are both observed by cameras. In such situations, for example, in endoscopy, the intensity of the image resulting from a white light illumination is preferably of the same order of magnitude as the intensity of the fluorescence image.
For operating rooms, in prior art, it has been provided to suppress by filtering in the ambient light the spectrum portion in which the fluorescent elements emit and to detect the fluorescence image with a camera provided with a filter which excludes any light outside the fluorescence spectral band. This method has many disadvantages. The filter must have outstanding performances in terms of extinction, since the ambient light-to-fluorescence ratio is huge (>106); the filtered lighting severely disturbs the colors perception; and each type of fluorescent element imposes a set of filters on the illumination system.